The present invention relates in general to data storage systems such as disk drives, and it particularly relates to a thin film read/write head for use in such data storage systems. More specifically, the present invention provides a new microelectromechanical (MEM) actuator incorporating a dual-stator design operated electrostatically in conjunction with a rotor to affect a fine positioning of the thin film magnetic read/write head. The substantial gain in the frequency response bandwidth greatly improves the performance and accuracy of the track-follow control for fine positioning of the thin film read/write head.
In a conventional magnetic storage system, a thin film magnetic read/write head includes an inductive read/write transducer mounted on a slider. The read/write head is coupled to a rotary actuator magnet and a voice coil assembly by a suspension and an actuator arm positioned over a surface of a spinning magnetic disk. In operation, a lift force is generated by the aerodynamic interaction between the read/write head and the spinning magnetic disk. The lift force is opposed by equal and opposite spring forces applied by the suspension such that a predetermined flying height is maintained over a full radial stroke of the rotary actuator assembly above the surface of the spinning magnetic disk. The flying height is defined as the spacing between the surface of the spinning magnetic disk and the lowest point of the slider assembly. One objective of the design of magnetic read/write heads is to obtain a very small flying height between the read/write element and the disk surface. By maintaining a flying height close to the magnetic disk, it is possible to record short wavelength or high frequency signals, thereby achieving high density and high storage data recording capacity.
The slider of the read/write head incorporates an air bearing surface to control the aerodynamic interaction between the read/write head and the spinning magnetic disk thereunder. Air bearing surface (ABS) sliders used in disk drives typically have a leading edge and a trailing edge at which thin film read/write transducers are deposited. Generally, the ABS surface of a slider incorporates a patterned topology by design to achieve a desired pressure distribution during flying. In effect, the pressure distribution on the ABS contributes to the flying characteristics of the slider that include flying height, pitch, and roll of the read/write head relative to the rotating magnetic disk.
In a conventional magnetic media application, a magnetic recording disk is comprised of several concentric tracks onto which magnetized bits are deposited for data recording. Each of these tracks are further divided into sectors wherein the digital data are registered. As the demand for large capacity magnetic storage continues to grow at an ever increasing pace, the current trend in the magnetic storage technology has been proceeding toward a high track density design of magnetic storage media. In order to maintain the industry standard interface, magnetic storage devices increasingly rely on reducing track width as a means to increase the track density without significantly altering the geometry of the storage media.
As the track width becomes smaller, this poses several mechanical and electrical problems to the operation of magnetic disk drives. One such problem lies in its actuation and control feature, which is critical to the operation of a magnetic disk drive. In order to appreciate the magnitude of this problem, it is necessary to further describe the control aspect of a typical magnetic read/write head. In a conventional magnetic disk drive, a read/write head includes a transducer mounted on a slider. The slider in turn is attached to a stainless steel flexure. The flexure and the load beam to which the flexure is attached together form a suspension arm. The suspension arm is then connected to an actuator arm, which is driven by a voice coil motor (VCM) to cause it to rotate at its mid-length about a pivot bearing. The suspension arm exerts an elastic force to counteract the aerodynamic lift force generated by the pressure distribution on the ABS of the slider. The elastic force together with the stiffness of the suspension arm controls the stability of the actuator arm with respect to the pitch, roll, and yaw orientations. With respect to the control feature of the magnetic disk drive, during each read or write operation, there are usually two types of positioning controls: a track-seek control and a track-follow control.
A track-seek control is typically commanded when data are to be retrieved from or new data are to be written to a particular sector of a data track. Electronic circuitry incorporating an embedded feedback control logic supplies a necessary voltage to the VCM to actuate it to drive the actuator arm, onto which the read/write head is attached, to a target track. Thus, a track-seek control performs a low-resolution or coarse positioning of the read/write head from one data track to another data track.
Upon the completion of a track-seek control, subsequent data operation is typically confined to within the target track. In the earlier stage of the magnetic storage technology, a typical data track is sufficiently wide so that small variations in the position of the read/write head resulting from external disturbances to the track-seek control plant do not cause the position of the read/write head to exceed the prescribed control error allowance. Therefore, no further control implementation in addition to the track-seek control is necessary. This type of control implementation is usually referred to as a single-stage actuation, which incorporates the VCM in the feedback loop to effect a total positioning of the read/write head.
As the track width reduces as a means to increase the track density and hence the storage capacity of magnetic disk drives, the foregoing single-stage actuation encounters a significant degree of difficulty, chiefly due to the excessive control error of the track-seek control using the VCM in the loop. In particular, a single-stage actuation using the VCM for low-resolution positioning of the read/write head is found to be inadequate because the resulting control error due to external disturbances such as inertial shock loading or noise sometimes could cause the read/write head to unintentionally position over the adjacent tracks, thus possibly causing a magnetic field disturbance of the existing data thereon. In the worst case, the data disturbances can result in a total erasure of data in the adjacent tracks after several repetitive write operations, or data corruption upon reading.
Moreover, the VCM employed in a single-stage actuation is typically subjected to a mechanical resonance at a low natural frequency in the range of 2000-3000 Hz due to the flexibility of the suspension arm assembly. The response of the servo-system further limits the frequency bandwidth to about 1500 Hz. As a result, this low frequency bandwidth imposes a severe penalty on the frequency response of the single-stage actuation system in such a manner that the track-seek control is unable to rapidly respond to a change in the position of the read/write head, thus causing a significant degradation in the performance of the magnetic disk drive.
To address this technical difficulty, it is recognized that in order to maintain the position of the read/write head in a manner that it follows a concentric path within a narrow track width of a target data track, necessary corrections to the motion of the actuator arm are required. This provision is made possible by a track-follow control, which uses a feedback on the track error signal to make an appropriate correction to the motion of the actuator arm so as to maintain the position of the read/write head to follow a concentric path of the target data track within a prescribed control error allowance. Thus, in the presence of external disturbances, variations in the position of the read/write head would not cause the position of the read/write head to significantly deviate from the target position in excess of the control error allowance.
To implement this track-follow control plant, a microactuator is frequently incorporated in the control feedback loop. Various types of microactuator have been proposed, including piezoelectric (PZT) actuators, electrostatic microelectromechanical systems (MEMS), and electromagnetic MEMS. By adjusting the voltage supplied to the microactuator, the track-follow control makes necessary corrections to the position of the actuator arm in the presence of external disturbances so that the read/write head is maintained to follow the target data track with a reasonable degree of precision. The implementation of a new, separate track-follow control for high-resolution positioning in addition to the usual track-seek control for low-resolution positioning as in the single-stage actuation is typically referred to as a dual-stage actuation, which constitutes the predominant control system employed in high capacity magnetic disk drives.
There currently exist a number of different microactuator designs in use for high-resolution positioning of read/write heads in high capacity magnetic disk drives. One such widely used conventional microactuator design is based on an electrostatically coupled rotor-stator concept. In principle, the rotor of the example conventional microactuator is physically connected to the outer stator by means of a plurality of elastic springs.
Moreover, the rotor is also electrostatically coupled to the outer stator by means of a plurality of radial spokes extending outwardly from the rotor and interleaving with the same plurality of similarly featured radial spokes extending inwardly from the stator. When a track-follow control plant commands a voltage to be sent to the example conventional microactuator in order to make a necessary correction to the motion of the actuator arm, an electrostatic potential field is induced within the radial spokes of the rotor and stator. By controlling the voltage polarity, an electrostatically attractive force can be generated between each pair of radial spokes between the rotor and stator. These circumferentially acting forces effectively result in a torque that causes a rotational motion of the rotor on which the slider containing the read/write head is mounted.
Notwithstanding the ability to achieve the track-follow control objective, the conventional electrostatically coupled rotor-stator microactuator design suffers a number of shortcomings that may offset the advantages it offers. These shortcomings may manifest into a number of problems as follows:
Because of the elastic spring connection of rotor to the stator and the mass of the rotor itself, the conventional microactuator of electrostatically coupled rotor-stator design possesses some natural frequencies of vibration. In particular, these natural frequencies are of low values because of the relatively substantial length of the elastic springs connecting the rotor to the stator. These low natural frequencies of vibration can easily be excited by an inertial force due to a sudden motion as commanded by the track-follow control, thereby rendering the microactuator susceptible to shock and vibration. Thus, in order to minimize this susceptibility to shock and vibration, the track-follow control may have to command a more gradual motion to reduce the inertial force loading. In so doing, the track-follow control performance may be degraded because of the reduced speed of actuation.
Furthermore, as the conventional microactuator of electrostatically coupled rotor-stator design is subjected to a typical excitation force during operation, the ensuing vibration of the rotor connected to the elastic springs manifests into an uneven oscillating forces at the spring supports on the outer stator. These forces may act transversely to the axis of the suspension arm assembly to which the stator is attached and present themselves as potential excitation forces to the suspension arm assembly. If these forces are sufficiently large and possess the excitation frequency near the natural frequency of the suspension arm, a resonant vibration of the suspension arm assembly would ensue, thereby causing an undesirable disturbance problem for the track-follow control system.
Yet another problem associated with the conventional electrostatically coupled rotor-stator microactuator design lies in its inherent non-linearity. The non-linearity is a manifestation of the force dependence on the voltage and the inverse dependence of the electrostatic force on the gap/engagement between the fingers. As in most physical systems, linearity is a highly desired virtue since it greatly simplifies the electromechanical conversion process. Furthermore, most modern control logics are built upon the premise of linearity in the system to be controlled. Hence, the non-linearity in the conventional electrostatically coupled rotor-stator microactuator design adds a significant degree of complexity in the operation and control of the conventional microactuator.
In light of the foregoing shortcomings with the conventional microactuator of electrostatically coupled rotor-stator design, it is recognized that a further enhancement in the microactuator design for a fine positioning of the read/write head is needed. Preferably, the enhanced microactuator would have a greater frequency response than the convention microactuator without adversely affecting the load beam vibration mode of the suspension arm assembly. Moreover, the enhanced microactuator design should have an enhanced linearity in order to reduce the complexity of the mechanical actuation.
In order to solve the foregoing difficulties, it is a feature of the present invention to provide a new enhanced microelectromechanical (MEM) actuator design for fine positioning of the read/write head during a track-follow control operation. The enhanced microactuator according to the present invention is designed to be used in a collocated dual-stage actuation servo system that substantially boosts the servo frequency bandwidth to enhance the track-seek and track-follow controls for an extremely high capacity magnetic storage devices. The present invention features an electrostatic MEM microactuator that is designed to rotate in response to a track-follow control voltage command.
According to a preferred embodiment, the electrostatic MEM microactuator of the present invention is comprised of a rotor electrostatically coupled to two stators arranged in an alternating manner. The stators are further comprised of a plurality of electrodes arranged perpendicularly along a plurality of stationary radial spokes, which are interleaving and oppositely disposed to the same plurality of moveable radial spokes formed on the rotor.
A track-follow control commands a voltage to be supplied to the rotor and stators, thereby inducing an electrical potential field to generate an electrostatically attractive force for each pair of radial spokes between each of the stator and the rotor. These circumferentially acting forces result in a torque that causes the rotor on which the slider is mounted to displace in a pure clockwise or counterclockwise rotation about the center of the flexure. The enhanced microactuator design of the present invention demonstrates the following improvements:
1. The use of two stators alternating with a rotor results in a greater elastic stiffness of the microactuator of the present invention due to the shorter spring length, thus substantially raising the natural frequencies of rotor. The increased natural frequencies translates into an improved frequency response of the microactuator of the present invention, thus enabling it to rapidly achieve a precise positioning of the read/write head;
2. The increased stiffness of the elastic connection of the rotor to the stators greatly diminishes the susceptibility of the microactuator of the present invention to shock and vibration, and further reduces excitation forces to the suspension arm assembly in the load beam modes;
3. Dividing the motor into quadrants to act in a pull hardxe2x80x94pull easy mechanism could enhance the motor linearity. In the current design, the doubling of the number of quadrants as a result of the circular symmetry of the microactuator of the preferred embodiment as compared to the conventional microactuator effectively enhances the linearity of the motor, thus reducing the complexity of the operation and control thereof.
4. The enhanced microactuator design of the preferred embodiment preserves the industry standard interface with a conventional suspension arm assembly without the necessity for a modification thereof, thus, rendering the performance of the enhance microactuator of the present invention virtually unaffected by various different form factors of the slider/suspension assembly such as Pico and Femto form factors.
A number of alternative embodiments are derived from the aforementioned novelties of the present invention. One such alternative embodiment includes a different physical arrangement of electrodes on non-equidistant radial spokes extending from the two stators with the outer stator closer to the rotor than the inner stator, thus resulting in combined electrostatically attractive forces between the radial spokes of the rotor and stators, which lead to a pull hardxe2x80x94pull easy arrangement instead on a pull only mechanism. As a result, the linearity of this microactuator could be further enhanced.
Another alternative embodiment of the present invention pertains to a microactuator employing a plurality of rotors and stators positioned relative to one another in an alternating arrangement. Thus, the resultant torque that causes a rotation of the slider is created by a combination of a series of forces acting between the rotors and stators.